Several attempts have been made in the prior art to remove and prevent escape to the atmosphere of the acid mist that rises above the upper part of electrolytic cells. The solutions devised may be classified into four groups:                a) systems that dilute the contaminated air;        b) systems that bring down the contaminants in the acid mist;        c) chemical or physical agents that try to prevent bubble from forming or that collect these bubbles to deposit them back into the electrolytic bath; and        d) systems that capture the mist at its source of emission and evacuate it towards a central decontamination system, with or without use of forced suction.        
In all these mechanisms for controlling acid mist results have been insufficient and/or have given rise to other problems. Those systems that dilute contaminated air typically comprise cross ventilation within the electrowinning plant. Bores are provided in the wall opposite to the inflow of fresh air, or even complete removal of said wall is provided, in order to extract acid mist out of the plant. However, heavy global thermal losses occur in the electrolytic process and the noxious effect on the workers, the facilities and the environment is not prevented.
Systems that reduce contaminants in the acid mist typically use sprinklers that send forth fine water drizzle that precipitate contaminants from the acid mist onto the cells and onto the plant floor. This is, however, a mere palliative that transfers contamination from aerial contamination to a diluted acid over the plant building and equipment.
The agents that try to prevent bubble formation include, among others, surfactants that decrease the surface tension of the electrolyte (and therefore, the size of bubbles generated in the anode), baffle plates that coalesce the bubbles, or else, balls, pellets or other inert floating particles that are incorporated into the acid bath and act as barriers to acid mist formation. These agents are used only as a complement of the other systems because they provide only a partial solution to the problem.
Regarding acid mist capturing systems, they typically consist of rigid or flexible covers of an electrolyte resistant material which are applied above the cells and are connected to a network of suction ducts that evacuate the mist towards a gas scrubber, plate filter, dehumidifiers and other devices in order to recover and/or carry out an environmentally friendly disposal of the acid and the contaminating substances contained in the acid mist. The idea is to generate a gentle, low-pressure suction that allows evacuation of the mist toward these ducts. The suction rate must be restrained to maintain moisture and temperature under the cover in the entire cell so as to avoid generation of crystals (salts) by over saturation of the aerosol droplets that are otherwise produced at a high rate and with cooling of the mist.
One of the best known and more widely used acid mist capture systems is one that consist of “high-energy hoods” that are placed over the anodes and cathodes and above the electrolytic bath, and comprise perforations in their bottom through which the mist is suctioned towards a centralized contamination handling system. The hoods, exemplified in Chilean patent application CL 247-1999 (Mella), have a rinsing system incorporated to keep the suction perforations free from formation of crystals that may obstruct said perforations. Still, the electrode supporting connector bars become corroded due to the thermal gradients produced when fresh air filters into the cells, in addition to other drawbacks. Moreover, hood operation must be accompanied by expensive automatic equipment to ensure accuracy and avoid damage during harvest of the cathodes, wherein said hoods are removed and subsequently replaced. During the harvest operation the electrolytic bath is necessarily exposed to the ambient and a gust of acid mist is produced that escapes to the surrounding atmosphere and requires the use of secondary ventilation systems.
Another of these acid mist capture systems consist of roofs or covers that are installed above the electrolytic bath surface and optionally above the electrodes themselves, above or below the electric connections, forming a substantially airtight seal over the bath. The mist is confined inside the volume formed between the electrolyte surface, the cell walls and the cover, and is sucked through suction ducts, either naturally or in forced fashion.
Chilean patent application No. 527-2001 (Vidaurre) discloses an improved container design for an electrolytic cell provided of several means for decontaminating acid mist. These means mainly comprise some flat covers that entirely enclose the upper plane surface of each container from the outer surroundings. They also comprise aerosol suction ducts mounted above the level of the electrolyte, which cross through at least a heightened front wall, and also along the side walls of the container, the latter formed either on the side walls themselves or mounted on preformed plastic moldings arranged on the side walls. These suction ducts in the cell are connected to a network of outer ducts for acid mist collection and to a central suction system. In addition, water sprinklers are provided to bring down the contaminated gases inside the container.
The generic cover of this Chilean invention comprises three sections of rigid and flexible plates, that is: a central longitudinal flexible (removable) cover and two preferably rigid and transparent lateral covers that allow conducting visual inspection of the electrodes. These covers are complemented with flexible seals between the central and lateral covers, which provide gaps to allow the inflow of moderate volumes of fresh air from the plant into the container. The cover is set up over a reticular structure that rests on the plane of the electrodes and the heightened front walls. To carry out acid mist extraction, the ducts in the at least one heightened wall and on the side walls of the container are connected to lateral longitudinal extraction ducts formed between longitudinally paired containers as a result of horizontal ledges that are molded on the outer faces of the container side walls. With the central suction system, pressure in the ducts is maintained somewhat below the atmospheric pressure in the plant, thus allowing the inflow of fresh air from the plant into the container at a low rate and in moderate volumes through the gaps in the flexible seals in order to ensure confinement of the mist under the cover.
During harvest and when the electric contacts are being periodically checked and cleaned, the longitudinal covers are removed. This adds an extra operation to the harvest, and makes the escape of acid mist to the surroundings inevitable, even if the central suction system is kept in operation.
U.S. Pat. No. 5,609,738 (Murray et al.) discloses a multi-element cover system applied below the electrode connections and above the surface of the electrolytic bath which does not require to be removed during the harvest and includes: a) flat caps that are placed against the anodes and span towards the contiguous cathodes; b) flexible plastic bands on the tank sides; and c) covers on the cell ends. The acid mist is evacuated naturally through the discharge weir for spent electrolyte, or via the overflow box. The free section above the weir duct generates a pressure differential and a natural suction that allows the mist to flow out, and it is also possible to use forced draft in the drainage system.
When this solution was applied to a real operation in a copper electrowinning facility in Chile, it could be established that due to the fact that the wetted weir mouth was placed at one end of the electrode row, the suction rate determined to avoid salt deposition in the weir mouth was not sufficient to conduct a uniform extraction of the mist contained throughout the cell, and the mist was dammed up between the anodes and the cathodes that were farther from the suction point. One of the problems caused by this situation was the attack (corrosion) on the metallic surface of cathode mother plates (pitting) by the acid mist, making detachment of the copper deposited on the cathodes very difficult. After a few months of use the poor results obtained forced to discontinue this system.
The present invention solves this and other problems of the prior art related to mist capture and removal in the way that shall be described hereinafter, with the benefit of keeping the acid mist permanently under control in a simple and effective manner throughout the entire cell, even when cathode harvest takes place, thereby accomplishing a uniform capture and removal of the same throughout the cell and requiring for this a very low energy, since good use is made of the energy of the aerosol itself that is being formed over the electrolytic bath. Moreover, with the system of the invention, the cathode harvest process is not altered with additional operations and is designed to be perfectly well adapted to cells existing at present in electrowinning plants.